1. Field of the Invention
The present invention relates to apparatus and methods used for separation of a mixed fluid such as a production fluid obtained in underground wells which is comprised of a mixture of oil and water. In one specific aspect, the invention provides for separation of the mixed fluid at a location outside of the wellbore. Water which is separated from the mixed production fluid is then transmitted to a second downhole location for reinjection into the producing formation.
2. Description of Related Art
Increasingly, fluid separation systems are being incorporated into oil production facilities. hydrocyclone-based separators are known which are capable of substantially separating a mix of two liquids having different densities into two streams of those constituent liquids. Gravity separators are also known in which an oil/water mixture within a separator pot is separated through natural gravitational forces so that the oil floats to the top of the pot and removed and the water is removed toward the lower end of the pot. Some composite or staged systems are known in which an initial separation of the mixed production fluid is accomplished by a gravity separator. Water separated from the production fluid by the gravity separator then has additional oil removed from it by parallel hydrocyclones.
Borehole separator arrangements are known for separation of production fluids. With these, a hydrocyclone-based separator is incorporated into the production tubing string and placed downhole. Locating the separator assembly itself within the wellbore in this manner permits the water to be removed while it is still downhole rather than producing excess water along with the oil produced. Further, the water separated by a separator which is located within the wellbore could potentially be reinjected into other portions of that wellbore such as into injection perforations. One disadvantage to this type of separation and reinjection arrangement is that the sizes of the separator assembly as well as the flow tubing into and out of the separator assembly is restricted by those which are capable of fitting within the wellbore casing diameter.
A few arrangements have been used wherein a separator assembly is located at the surface of the wellbore outside of the opening of the well so that the wellbore diameter does not restrict the size of the separator assembly and the associated flow tubing. These surface-based separator assemblies include a gravity separator placed in series with parallel hydrocyclone separators. Production fluid is pumped to the surface of the well and from there into the separator assembly where an initial separation of the production fluid into separated oil and separated water is performed by the gravity separator. Following the initial separation, the stream of separated water is transmitted through the two hydrocyclones for removal of residual oil. The residual oil removed by the hydrocyclones is then added to the separated oil for collection. Surface based systems such as this typically draw production fluid from each of several wells within a field of wells and direct all of the production into a single manifold. One large separator unit is integrated downstream of the manifold as part of the production flowline. Such a system is described in a recent publication entitled "Subsea Water Separation" by Velle et al. However, control of this single separator and hydrocyclone assembly is complex and, in most cases, requires electrical signaling to properly open and close valves to regulate the system. Specifically, a control valve is associated with the oil/water pot of the gravity separator which regulates the level of the oil/water interface within the pot. Regulator valves are required to bring the hydrocyclones on and off line in order to maintain their flow rates within the operating band.
Unfortunately, operation of the single separator system is also dependent upon its receipt of an adequate amount of composite flow from the multiple wells. The relationship between the flow rate and operation of the hydrocyclone and separator assembly is commonly measured by the turndown ratio for the separator assembly. The turndown ratio is the ratio of the separator assembly's maximum capacity to its minimum capacity required for operation. When production is obtained from multiple wells rather than a single well, the possibility of falling below the minimum required capacity is increased. If production from some of the multiple wells were to cease or be significantly reduced, flow rate into the single separator assembly might become inadequate to ensure proper separation.
A related problem exists with surface-based central separator arrangements used in subsea systems where the separator assembly is located on the sea bed. Upon separation of the production fluid, separated oil is transported to the surface via a production line while separated "clean" water is released into the sea. Unfortunately, release of produced clean water into the sea can create problems for and impose additional costs upon petroleum producers. Current regulations require that released fluid contain less than 40 parts per million (ppm) of oil. The well operator or supervisor is obligated to monitor the levels of oil in the released fluid and make reports of its content. Oil level monitors must be installed to measure the amount of oil present in the discharge. Typically, redundant monitors are required to insure accuracy and to guard against failure of a single monitor.
Additionally, it is noted that the use of oil/water separation equipment has traditionally been associated with late stage production from wells. Therefore, these assemblies have been emplaced in prior art wells after production through traditional production strings has become uneconomical. However, the initial production string must first be pulled from the well in order to install the separation assemblies, particularly those separation assemblies which must be located within the wellbore.
The methods and apparatus of the present invention overcome the drawbacks inherent in the prior art.